Causes of encephalitis

It's not always clear what causes encephalitis, but it can be caused by:

viral infections – several common viruses can spread to the brain and cause encephalitis in rare cases, including the herpes simplex virus (which causes cold sores and genital herpes) and the chickenpox virus

a problem with the immune system (the body's defence against infection) – sometimes something goes wrong with the immune system and it mistakenly attacks the brain, causing it to become inflamed

bacterial or fungal infections – these are much rarer causes of encephalitis than viral infections